Field of the Invention
The invention relates to a nuclear plant having a containment shell and having at least one pressure relief line, which passes out of the containment shell and can be sealed by a shut-off valve, and through which a pressure relief flow can flow during relief operation when the shut-off valve is open.
During fault and emergency situations in nuclear power plants, comparatively large amounts of hydrogen and carbon monoxide and water vapor may be released in the safety container, also known as a containment, a confinement or a containment shell, which hermetically shields the atmosphere in the interior from the external environment.
Without counter-measures, under some circumstances it is possible for flammable gases to accumulate in the containment shell atmosphere to such an extent that they can form detonable mixtures. Further, in particular if water vapor is released from the cooling circuit, overpressures exceeding the pressure specifications of the safety enclosure may occur.
In many nuclear power plants, various systems are already installed for eliminating hydrogen and for limiting the overpressure and relieving the pressure of the containment in emergency situations. In general, systems of this type are designed, configured and set in operation independently of one another.
In this connection, the prior art includes for example catalytic elements, also known as catalytic recombiners depending on the construction, which recombine hydrogen (H2) contained in a flow of gas with oxygen (O2) in a catalytically driven, flameless, exothermic reaction to form water (vapor) (H2O). The same applies to the recombination of carbon monoxide (CO) with oxygen (O2) to form carbon dioxide (CO2). For effective H2/CO elimination and to prevent unacceptable, locally critical concentrations, a number of recombiners are generally arranged throughout the containment.
Further, venting systems are known, by which overpressure states can be made manageable by releasing containment atmosphere into the environment, generally with filtering. Passive systems are preferred, which are driven by the overpressure in the containment itself. However, there are also variants in which the pressure relief flow from the containment is activated or promoted by electrically-driven conveyor fans and the like.
All of the variants have pressure relief lines which pass through the containment shell and which are sealed by at least one blocking valve during normal operation of the nuclear plant. For the desired pressure relief, the respective blocking valve is opened, in such a way that the gas/vapor mixture which is at an overpressure can flow out from the interior of the containment via the pressure relief line to the outside, where it is generally released into the environment after multi-stage filtering, purification and drying to retain radioactively contaminated liquids, particles and aerosols.
In certain scenarios, considerable H2 and/or CO formation combined with significant pressure build-up is possible, necessitating early pressure relief of the containment. As a result of the early pressure relief, it is conceivable that the measures for H2/CO elimination in the containment will not yet have taken full effect.
Thus, in unfavorable cases, it should be assumed that ignitable H2/CO concentrations are present in the vapor/air atmosphere in the pressure relief flow, also known as the vent gas flow, venting flow or vent flow for short. If the vent gas subsequently passes through non-temperature-controlled pipelines and filtering devices, the resulting vapor condensation leads to a further increase—for example doubling—in the concentrations of the flammable gases. As a result, ignitable and even detonable mixtures may be formed, which if ignited cause considerable flame accelerations which place the integrity of the exhaust and retaining devices at risk and can thus lead to highly undesirable environmental pollution and contamination if allowed to occur.
The venting systems are therefore permanently heated for example to prevent initial condensation, or else configured to be set in operation only after prior H2 elimination in the containment. Recombiner systems in the containment are accordingly configured in such a way in terms of power that early H2 elimination takes place using a plurality of recombiners and a largely H2-free atmosphere can thus be achieved before the start of the pressure relief in a number of conceivable emergency scenarios. In addition, the recombiners thus far are predominantly arranged in the primary convection paths. Typically, a high number of for example 20 to 100 recombiners or more are provided, leading to an integral recombiner through-flow power of for example 0.3 to 0.6 or more of the total volume of the containment atmosphere per hour, referred to herein as the air exchange number. Thus for example for a containment volume of 50,000 to 70,000 m3 a recombiner throughput power of 15,000 to 40,000 m3/h or more is required. Nevertheless, in the aforementioned critical scenarios the H2 elimination may not always be ensured in good time before the venting is introduced.